The "Write Yourself a Git" (WYAG) guide explains how to build a simple version of Git in Python. Here’s a step-by-step summary of the key concepts and commands to implement your own Git-like system:

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1️⃣ Initialize a Git Repository

Every Git repository has a .git directory to store all objects and metadata.
Command to implement:

def init(repo_path):
    os.makedirs(repo_path + "/.git", exist_ok=True)
    os.makedirs(repo_path + "/.git/objects", exist_ok=True)
    os.makedirs(repo_path + "/.git/refs", exist_ok=True)
    with open(repo_path + "/.git/HEAD", "w") as f:
        f.write("ref: refs/heads/master\n")

Run:

python git.py init

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2️⃣ Storing File Content as an Object

Git stores files as blob objects in a compressed format.
Command to implement:

import hashlib, zlib

def hash_object(file_path):
    data = open(file_path, 'rb').read()
    obj = b'blob ' + str(len(data)).encode() + b'\x00' + data
    oid = hashlib.sha1(obj).hexdigest()
    
    obj_path = f".git/objects/{oid[:2]}/{oid[2:]}"
    os.makedirs(os.path.dirname(obj_path), exist_ok=True)
    
    with open(obj_path, "wb") as f:
        f.write(zlib.compress(obj))
    
    return oid

Run:

python git.py hash-object file.txt

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3️⃣ Retrieving Stored Objects

Retrieve and decompress stored objects from .git/objects/.
Command to implement:

def cat_file(oid):
    obj_path = f".git/objects/{oid[:2]}/{oid[2:]}"
    with open(obj_path, "rb") as f:
        data = zlib.decompress(f.read())
    print(data.split(b'\x00', 1)[1].decode())

Run:

python git.py cat-file <object-hash>

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4️⃣ Storing Directory Structure as a Tree

Git tracks directories using tree objects, which contain references to blob objects.
Command to implement:

def write_tree():
    entries = []
    for file in os.listdir("."):
        if os.path.isfile(file):
            oid = hash_object(file)
            entries.append(f"100644 {file}\x00{bytes.fromhex(oid)}")
    
    tree_data = b"tree " + str(sum(len(e) for e in entries)).encode() + b"\x00" + b"".join(entries)
    tree_oid = hashlib.sha1(tree_data).hexdigest()
    
    obj_path = f".git/objects/{tree_oid[:2]}/{tree_oid[2:]}"
    os.makedirs(os.path.dirname(obj_path), exist_ok=True)
    
    with open(obj_path, "wb") as f:
        f.write(zlib.compress(tree_data))
    
    return tree_oid

Run:

python git.py write-tree

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5️⃣ Creating a Commit Object

Commits store metadata and reference a tree object.
Command to implement:

def commit_tree(tree_oid, message, parent=None):
    commit_data = f"tree {tree_oid}\n"
    if parent:
        commit_data += f"parent {parent}\n"
    commit_data += f"author User <user@example.com>\n"
    commit_data += f"committer User <user@example.com>\n\n"
    commit_data += message + "\n"
    
    commit_obj = b"commit " + str(len(commit_data)).encode() + b"\x00" + commit_data.encode()
    commit_oid = hashlib.sha1(commit_obj).hexdigest()
    
    obj_path = f".git/objects/{commit_oid[:2]}/{commit_oid[2:]}"
    os.makedirs(os.path.dirname(obj_path), exist_ok=True)
    
    with open(obj_path, "wb") as f:
        f.write(zlib.compress(commit_obj))
    
    return commit_oid

Run:

python git.py commit-tree <tree-hash> -m "First commit"

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Summary of Commands

Command	Description
init	Initialize a repository
hash-object <file>	Store a file as an object
cat-file <object-hash>	Retrieve object contents
write-tree	Save the directory structure
commit-tree <tree-hash> -m "message"	Create a commit

This covers the basic Git internals 🚀